1. Field of the Invention
The present invention relates to a camera device having a posterior filter, and more particularly, to a camera device which has a posterior filter residing in a lens mount or its vicinity.
2. Prior Art
A camera device, which is sometimes used along with a sharp-cut filter, a neutral density (ND) filter, a color conversion filer, a color correction filter, an infrared cutoff filter for a CCD, or the like, is loaded with a filter ahead of an objective lens, or closer to an object, and this configuration of the camera device is advantageous in that the filter can be more easily attached to and detached from the camera device while it is accordingly unavoidable that the filter is considerably greater in diameter, and with a wide-angle lens having an almost 180-degree angular field, a frame of the filter is caught in a range of the angular field.
In order to overcome the above-mentioned disadvantages, usually an improved lens assembly used along with a posterior filter has been employed which is loaded with the filter in a location in or around a mount of a wide-angle lens. Such a detachable posterior filter incorporated lens assembly for a camera device is dedicated to a zoom lens and is comprised of a lens barrel, imaging optics therein, an imaging means located behind the imaging optics, and a displacement means for moving optical elements such as an optical filter along an optical path between the imaging optics and the imaging means (e.g., see Patent Document 1 as identified with Japanese Patent No. 2001-255444).
Another prior art embodiment is a camera lens assembly which is comprised of a lens barrel having a base plate at its bottom portion, a lens holder disposed in an optical axis within the lens barrel to retain groups of lenses, a fixed aperture stop, a fastening ring, and an infrared cutoff filter in positions and having cam pins in its outer circumferential surface, and an active focusing barrel, for manual adjustment, which is attached to the lens barrel and is provided with cam grooves respectively mated with the cam pins of the lens holder, and the active focusing barrel, while being revolved, permits the follower lens holder to linearly move along the optical axis by means of a cam mechanism consisting of the cooperative cam pins and cam grooves without rotary motion, thereby obtaining an image without angular and other deviations (e.g., see Patent Document 2 identified with Japanese Patent No. 2002-350701).
The cited embodiment as disclosed in Patent Document 1 facilitates attachment and detachment of the infrared cutoff filter through a simple manipulation. However, the infrared cutoff filter resides in a static portion of the lens barrel, and hence, forward movement of the imaging optics for the focusing results in the imaging optics having its most posterior plane spaced far from the infrared cutoff filter.
In such a situation, especially, when an imaging lens is a wide-angle lens, light flux exiting the most posterior plane of the imaging optics diverges at a great angle, and the infrared cutoff filter must have a diameter sufficiently large to avoid shading caused by itself. As a consequence, the displacement means of the optical elements should accordingly be large enough, and this is why the downsizing of the detachable posterior filter incorporated camera device is hard to attain.
The additional cited embodiment as disclosed in Patent Document 2 has its active focusing barrel revolved to activate the cam mechanism of the cam pins and cam grooves and let the follower lens holder linearly move along he optical axis together with the groups of lenses, the fixed aperture stop, the fastening ring, and the infrared cutoff filter retained by the lens holder without rotations of those retained components, thereby effectively avoiding the angular and other deviations of an image.
To keep all the components retained by the lens holder unrevolved, however, the linear movement of the lens holder along the optical axis should rely only on the cam mechanism of the cam pins and cam grooves mated with one another. This unavoidably causes the lens barrel to get bulky because several cam barrel members should telescopically overlap one another to let the groups of lenses move in various manners and directions relative to the optical axis, especially as in the case of the zoom lens assembly.
The camera device set forth in Patent Document 2 may be modified by providing a helicoidal portion in the fixed barrel(s) and the lens holder, respectively, to avoid using additional components for the barrel assembly. In this way, the barrel assembly can avoid getting bulky while enabling the groups of lenses to move differently relative to the optical axis, but instead, the lens holder and the components retained by the same are forced to revolve. Then, although a revision of the infrared filter permits the loading and unloading of the same in a direction perpendicular to the optical axis to ensure attachment of the infrared cutoff filter despite a simplified device design, the revolved lens holder causes an angular displacement of the direction of the attachment, which will be a cause of a difficulty in loading and unloading the infrared cutoff filter, and will also be prone to impede the loading and unloading of the same because of elements such as a signal propagation bump located in the posterior of the camera lens assembly.
The present invention is made to present a solution to the aforementioned problems in the prior art camera lens technology, and accordingly, it is an object of the present invention to provide a camera device having a posterior filter which is of a compact design without a cam barrel(s) and is capable of loading and unloading optical elements such as the posterior filter in a simplified manipulation by virtue of a fixed orientation of a filter holder.